Direct translations:
* נישט פֿאַרגעסן (nist fargesn): This is the most literal translation, meaning "not forget."
* פֿאַרגעס נישט! (farges nisht!): This is a more forceful way of saying "don't forget!"
* נישט קיינמאָל פֿאַרגעסן (nist kaynmal fargesn): This means "never forget" literally, emphasizing the duration.
More idiomatic expressions:
* זאָל עס נישט פֿאַרגעסן ווערן (zol es nisht fargesn varen): This means "May it not be forgotten," which is a more poetic way to express the sentiment.
* האלט עס אין זכּרון (halt es in zikhron): This means "keep it in memory."
* לאָז עס נישט פֿאַרשוונדן ווערן (loz es nisht farshvndn varen): This means "don't let it disappear," which implies the importance of remembering something.
The best option for you will depend on the context of your statement and the emotional impact you want to create.